As you know, I'm kind of obsessed with the geographical location of DC's fictional cities (as is my friend Siskoid). So when I come upon actual data in an actual DC story, you know I'm paying attention.
In Batman: Gotham Knights #1 (2000), Dick Grayson is tolling down the highway on his bike, and we see the following sign:
After taking Robin's call, he does a quick U-turn and...
We should acknowledge the following caveats: A) These are road signs, so this is driving distance, not "as the crow flies" distance, and there can be some considerable differences based on how much meandering various highways do. B) Many highway distance signs are notoriously poor approximations C) This is the DC Universe, so of course things there may not be located exactly where they are on our Earth...(particularly if you buy the Busiek Hypothesis. In Avengers/JLA, Kurt Busiek revealed that the DC Earth was larger than the Marvel Earth, and hence had more surface area, and thus more room for all these fictional towns.)
So, given those limitations, these signs tell us:
**Bludhaven and Gotham City are about 40 miles apart.
**Gotham is between Boston and Baltimore. And the distances are right: Google Maps has the driving distance between Boston and Baltimore at approximately 400 miles. So the signs, which have Gotham 100 miles from Boston and 300 miles from Baltimore, make sense.
**So, given that Gotham has a port, and assuming that it's not in the middle of the ocean; that's it's in a generally southwesterly direction from Boston; given that it's about 300 miles from Baltimore, and 100 miles from Boston; and moving little markers around on Google Maps...
We can conclude that Gotham City is most likely in southern Connecticut (with a slim possibility that it might be in Rhode Island.
Next up: finding a story where someone is driving between Metropolis and somewhere...